Astrophile: Binary stars that form like fraternal twins

One star per dusty disc is not always enough, it seems: if that disc fragments, a smaller spin-off star can build itself

(Image: Bill Saxton, NRAO/AUI/NSF)

By Rebecca Boyle

Object&colon; Binary starsFormation mechanism&colon; In the same stellar nursery

Protostar CB230 IRS1 never let anyone forget that he was the older twin. Although he and his brother were born of the same swirling mass of gas and dust, IRS1 knew that he coalesced first. The second star in the binary duo formed after the spinning disc fragmented, and congealed into a smaller star. And like human fraternal twins, though they were born in the same environment, the pair will grow up to look very different. The bigger twin will die first, and if the pair stay close enough throughout their lives, it could even turn into a supernova.

Astronomers have debated how binary stars, which make up about half of all sun-like stars in the galaxy, form and grow up. Some argue binaries are like twins, whether identical – beginning their existence as one entity and dividing in two early on – or fraternal – both forming from scratch in the same proto-stellar disc. Others think they’re more like lovers who found each other later in life, migrating across interstellar space until gravity joins them for good.

Advertisement

Now astronomers have snapped their first-ever baby photos of binary stars, showing that the twin model is correct, at least sometimes. The protostars in question won’t have finished forming for another million or so years, and only possess about 20 per cent of their ultimate mass, but their prenatal conditions offer a compelling explanation for the formation of binary systems in general.

“They are big and puffy and cute, and they’re in a nursery, as it were, for stars. If you are looking at the nursery, you are seeing how they first formed,” says Leslie Looney at the University of Illinois at Urbana-Champaign. “If you see a binary there, they formed together. You are looking at the first instance of formation.”

Bonus star prize

Looney and his colleagues used the recently upgraded Karl G. Jansky Very Large Array in New Mexico to study three protostars about 1000 light years away in the constellation Cepheus. In two cases, the VLA’s improved resolution enabled them to see the cold disc of gas surrounding a nascent star, along with a surprise&colon; a second bright clump, representing another natal star.

The team concludes that the protostellar disc somehow fragmented to form a second object. It’s unlikely that another, unrelated star happened to join the family in the same disc on the same plane. Because the objects are different sizes, it’s also unlikely that a single star formed first and then split, says John Tobin at the National Radio Astronomy Observatory in Charlottesville, Virginia.

Joel Tohline of Louisiana State University in Baton Rouge, who specialises in star formation, says the findings make a strong argument in favour of fraternal-twin formation. But he says it does not rule out the fission hypothesis – identical-twin formation. In that scenario, a single star spins faster as it collapses inward – like an ice skater whose spin speeds up as she pulls her arms to her sides – and the disc elongates, forming a peanut shape that splits in half.

By Jove!

“These are two distinctly different ways of fragmenting the central object,” Tohline says. “How can you really differentiate whether that happened by fission or by disc fragmentation? I don’t think it’s possible.”

To clear things up, Tobin and colleagues plan to survey a total of 75 protostars. Whether the stars are fraternal or identical twins could matter for their eventual demise. Fraternal twin stars are different sizes, and the more massive of the two will die first. If it’s not big enough to go supernova on its own, it will quietly eject its outer atmosphere and cool into a white dwarf. Later, when its lower-mass twin starts to age and puff out, it could start to gather up some of this hand-me-down material. If the older white dwarf takes in enough of this material to grow heavier than 1.4 solar masses, the physical limit for white dwarf stability, it could turn into a type Ia supernova, Looney says. But this could also happen in binary systems that form in other ways, he adds, as long as they end up close together 10 billion years or so after their conception.

The team would also like to know what leads to the formation of a second star – or even a giant planet – in the same disc. Their findings will shed more light on how stars are born, and perhaps the many family structures they can take, Looney says. He thinks the disc fragmentation that forms a second star might even have occurred in our solar system, with Jupiter the result.